solve problem Letter Case Permutation

Author: P-ppc

README.md

@@ -95,6 +95,7 @@ All solutions will be accepted!
 |697|[Degree Of An Array](https://leetcode-cn.com/problems/degree-of-an-array/description/)|[java/py/js](./algorithms/DegreeOfAnArray)|Easy|
 |671|[Second Minimum Node In A Binary Tree](https://leetcode-cn.com/problems/second-minimum-node-in-a-binary-tree/description/)|[java/py/js](./algorithms/SecondMinimunNodeInABinaryTree)|Easy|
 |242|[Valid Anagram](https://leetcode-cn.com/problems/valid-anagram/description/)|[java/py/js](./algorithms/ValidAnagram)|Easy|
+|784|[Letter Case Permutation](https://leetcode-cn.com/problems/letter-case-permutation/description/)|[java/py/js](./algorithms/LetterCasePermutation)|Easy|
 
 # Database
 |#|Title|Solution|Difficulty|

algorithms/LetterCasePermutation/README.md

@@ -0,0 +1,2 @@
+# Letter Case Permutation
+This problem is easy to solve by set

algorithms/LetterCasePermutation/Solution.java

@@ -0,0 +1,26 @@
+class Solution {
+    public List<String> letterCasePermutation(String S) {
+        Set<String> res = new HashSet<String>();
+        
+        for (char c : S.toCharArray()) {
+            String s = String.valueOf(c);
+            if (res.size() == 0) {
+                res.add(s.toUpperCase());
+                res.add(s.toLowerCase());
+            } else {
+                Set<String> temp = new HashSet<String>();
+                for (String v : res) {
+                    temp.add(v + s.toUpperCase());
+                    temp.add(v + s.toLowerCase());
+                }
+                res = temp;
+            }
+        }
+        
+        res.add(S);
+        
+        List<String> result = new ArrayList<String>();
+        result.addAll(res);
+        return result;
+    }
+}

algorithms/LetterCasePermutation/solution.js

@@ -0,0 +1,24 @@
+/**
+ * @param {string} S
+ * @return {string[]}
+ */
+var letterCasePermutation = function(S) {
+    let res = new Set()
+    
+    S.split('').forEach(c => {
+        if (res.size === 0) {
+            res.add(c.toUpperCase())
+            res.add(c.toLowerCase())
+        } else {
+            temp = new Set()
+            res.forEach(v => {
+                temp.add(v + c.toUpperCase())
+                temp.add(v + c.toLowerCase())
+            })
+            res = temp
+        }
+    })
+    
+    res.add(S)
+    return Array.from(res)
+};

algorithms/LetterCasePermutation/solution.py

@@ -0,0 +1,21 @@
+class Solution(object):
+    def letterCasePermutation(self, S):
+        """
+        :type S: str
+        :rtype: List[str]
+        """
+        res = set()
+
+        for c in S:
+            if len(res) == 0:
+                res.add(c.upper())
+                res.add(c.lower())
+            else:
+                temp = set()
+                for j in res:
+                    temp.add(j + c.upper())
+                    temp.add(j + c.lower())
+                res = temp
+        
+        res.add(S)
+        return list(res)